THE INTERMEDIATE FOODS OF THE SEA 



These floating very small sea plants occur in all localities, but 

 they are naturally much more abundant in some places than 

 in others. They are subject to great seasonal fluctuations in 

 their numbers, and they become less common, many of them 

 entirely disappearing, toward mid-ocean. 



They are so very small that, although their presence may 

 convert the sea water into a thin Hving- vegetable soup, special 

 adaptations are necessary to enable animals to feed upon them. 



These adaptations are along three main lines. 



Many animals of a structure very similar to that of these 

 plants, some almost as small but others larger, live among 

 them entangling them in net-works of slender sticky threads pro- 

 jected from their bodies. Such are the oceanic foraminifera 

 and the radiolarians. Some of the peridinians, too, are incapable 

 of synthesizing inorganic into organic substances, and therefore 

 live upon the other little floating plants in the same way that 

 rusts' and blights live upon the leaves of plants on land. 



Some animal types have given rise to very small forms which 

 are able to pick out the little clusters of minute plants from 

 the sea water. It is rather curious that the two animal groups 

 to which, outside of the vertebrates, aU the giants of the sea 

 belong, the crustaceans and the molluscs, should have been the 

 ones to produce the vast bulk of small creatures which feed 

 upon the little plants. 



Most numerous in kinds and numbers are the very small 

 crustaceans of many different sorts which at certain places and 

 at certain seasons occur in myriads and are present in greater 

 or lesser abundance almost everywhere, and the very young 

 stages of many of the larger ones — crabs, lobsters, shrimps, 

 etc. Just as on land the insects are the chief intermediates 



1 60 



